Method for forming refractory tubing

ABSTRACT

Refractory tubings, either in amorphous, polycrystalline or single crystal form, are made by moving a preformed tubing of a refractory material and a heated zone relative to each other, the heating zone providing sufficient heat to melt through the tubing and form a molten ring which is continuously advanced through the tubing. The heat source may be located externally or internally of the tubing wall or in both positions. The tubings may be formed as single crystals by using appropriate seeds; and by controlling the rate of movement of the tubing sections on either side of the molten ring the wall thickness and diameter of the final tubing may be adjusted.

This application is a continuation-in-part of my application Ser. No. 509,802 filed Mar. 19, 1975, which is a divisional application of U.S. Ser. No. 318,911 filed Dec. 27, 1972, now U.S. Pat. No. 3,943,324 which in turn was a continuation-in-part of Ser. No. 97,603 filed Dec. 14, 1970, now abandoned.

This invention relates to a method for forming refractory materials into the form of tubing, and more particularly to forming refractory tubing in amorphous, single crystal or polycrystalline forms.

The term "refractory" is used hereinafter to designate materials which have relatively high melting points and which may or may not be excessively corrosive. The term is meant to include amorphous and crystalline materials, including glass, single crystal and polycrystalline forms; compounds such as alumina, silica thoria, zirconia, ytteria, etc.; intermetallics such as gallium arsenide and pseudobinary compounds such as AlAs-GaAs; as well as elements such as germanium and silicon. Any refractory suitable for this invention must be capable of existing in a molten state whether under ambient conditions or controlled environment.

Although a number of refractories have been made into tube forms by conventional powder processes, at present alumina is the most important of these refractories where high-temperature strength, high electrical resistivity and chemical inertness are required. Therefore, alumina may be taken as exemplary in discussing prior art and utility. It is, of course, possible that other refractory materials, particularly the oxides of yttrium, zirconium and thorium, can be used in a manner similar to that in which alumina is now employed.

An important application for alumina tubing is enclosures for high-pressure sodium-halide lights. The emitted light from a high-pressure sodium-halide (normally sodium iodide) light is more pleasing than the yellow light emitted by low-pressure sodium vapor lamps. Moreover, these lamps are smaller and more efficient than alternative lamps such as mercury vapor lamps or fluorescent sources. At the increased operating temperatures characteristic of the high-pressure sodium lamps, the gases become too corrosive to permit the use of previously acceptable vitreous silica enclosures. These more severe conditions have led to the use of alumina enclosures which are formed either by sintering the alumina in the desired configuration, or by adaptation of the Czochralski crystal pulling technique.

Even with the most sophisticated sintering techniques, the sintering process rarely produces materials which attain full or theoretical density. Such failure to attain full density means that alumina tubes or envelopes formed by sintering alone probably have residual porosity which provides light scattering sites, thus detracting from the efficiency of any lamps formed from the sintered tubing.

To minimize porosity in materials produced by this prior art sintering techniques, it is necessary to fire the pieces in an atmosphere made up of a gas which has a sufficiently high solubility and mobility for diffusion of the gas entrapped in closed-off pores out of the sintered material. Such an atmosphere is, for practical purposes, limited to hydrogen. Alternatively, the sintering may be done in a vacuum. Thus, in the prior art processes, there is virtually no freedom to select ambient atmospheres to maximize purification of the final tubing material or to attain other desirable results such as the ability to adjust the valence state of intentionally added dopants, additives or of residual impurities.

It may be desirable to be able to form such tubing of a single crystal. If the material is not optically isotropic, for example materials having hexagonal crystal structures, the presence of a plurality of different grain boundaries in the optical path will degrade the potential image quality of transmitted light. Such grain boundaries may be effectively eliminated by forming the tubing as a single crystal. This is not the case with cubic crystals which are optionally isotropic. However, grain boundaries in most materials act as concentrators as well as highmobility paths of impurities. Formation of refractory tubings as single crystals has several additional important advantages. Thus, for example, it is possible to eliminate, reduce or control the stresses in as-grown tubing by eliminating stresses due to thermal expansion anistropy between the grains. Forming single-crystal tubing can also provide a crystallographic orientation that is favorable for relaxing as-grown stresses.

The adaptation of the Czochralski method of growing crystals to the formation of tubing by pulling the tubing from a hot melt contained in a hot crucible presents the serious disadvantage of introducing contaminants into the tubing from the crucibles. Such contaminants may interact with the active gases within the lamp enclosure serving as "getters" for these small quantities of gases or they may increase the total absorption across the emitted spectrum of the lamp thus decreasing its efficiency. Ambient atmospheres must be limited to those which do not result in degradation of the crucible. Finally, there are many materials for which no known crucible material exits.

The principal disadvantages of the prior art methods--failure to attain full density with the resulting undesirable degree of porosity, introduction of impurities, and restrictions imposed on processing atmospheres--are minimized or eliminated by the method of this invention.

It is therefore a primary object of this invention to provide an improved method for forming amorphous or crystalline refractory tubing. It is another object to provide a method of the character described which makes possible the forming of refractory tubings of materials exhibiting full density or near full density and hence of materials in which light scattering sites are reduced to a minimum. Another object is the providing of such a method which makes it possible to make refractory tubing of extremely high purity. It is yet another object of this invention to provide a method for forming refractory tubing containing additives, the valence state of which may be adjusted. An additional object is to provide a method of the character described which makes possible, if desired, formation of refractory tubings in singlecrystal form. It is yet another object of this invention to provide a method of forming refractory tubing, the cross sectional configuration and wall thickness of which may be varied. It is another object of this invention to provide such a method which is applicable to a wide range of high-temperature materials including those normally considered to be too corrosive to be contained in crucibles. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

By the method of this invention there is formed a refractory tubing exhibiting improved physical properties. This method comprises the steps of providing a preformed refractory tubing blank; providing around the tubing blank a concentrated heating zone of radiant energy of substantially uniform intensity thereby to form in the tubing a molten ring extending through the wall of the tubing and being of essentially uniform height through the tubing, the height being such that the forces of surface tension and gravity maintain the molten ring connected to solid sections on either side of the molten ring; and controllably effecting relative translational movement between the refractory tubing and the heating zone thereby to advance the molten ring through the tubing. The solid sections on either side of the molten ring may be rotated in the same or opposite directions; and the tubing may be formed as a single crystal by using appropriately configured seed crystals.

When each end of the tubing is separately held and when separate moving means are associated with one or both of the two solid sections (one on each side of the molten ring) it is possible by adjusting the speed of one of the moving means relative to the speed of the other moving means or the speed at which the heating means are moved to control the thickness of the final tubing wall. Other method modifications are also possible.

Apparatus suitable for carrying out the method of this invention may comprise means to provide a heating means external of a refractory tubing, internal of the refractory tubing or a combination of both, and means to move the tubing and heating zone relative to each other. It may also include means to provide different rates of translational and rotational motions for each solid section on the two sides of the molten ring as well as means to provide a controlled fluid atmosphere around the system. Radiant energy supplied to the heating zone is preferably laser energy.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which

FIG. 1 is an enlarged longitudinal cross section of the tubing during formation wherein the finished refractory tubing has essentially the same wall thickness as the preformed feed tubing;

FIG. 2 is an enlarged longitudinal cross section of the tubing during formation wherein the finished refractory tubing has an attenuated wall thickness;

FIGS. 3 and 4 are typical cross sections taken in a plane normal to the tubing axis;

FIG. 5 illustrates in diagrammatic perspective the use of a ring to start the melting of the tubing at one end;

FIGS. 6 and 7 illustrate in diagrammatic perspective the use of a flat plate to start the melting of the tubing at one end;

FIG. 8 illustrates the use of a rod to start the melting of the tubing at one end;

FIGS. 9-12 illustrate the steps of initiating the formation of a single crystal refractory tubing in accordance with one embodiment of this invention using one combination of feed tube and pick-up seed crystal configurations;

FIGS. 13-17 illustrate other combinations of feed tube and pick-up seed crystal configurations;

FIG. 18 is a representation, partly in cross section, of suitable embodiment of apparatus for carrying out the method of this invention in which the heating zone is provided through the use of an incandescent heater heated by an r.f. coil;

FIG. 19 is a representation, partly in cross section, of another type of incandescent heating means and the use of radiation shielding within the tubing to control the direction of heat flow;

FIGS. 20-22 illustrate the use of an arc, a filament, and an r.f.-coupled graphite rod, respectively, as internally located heating means;

FIG. 23 is a top plan view of an optical system using a laser as the means for providing the heating zone;

FIG. 24 is a cross sectional view through the optical system of FIG. 23 showing the spatial locations of the mirrors used;

FIG. 25 is a top plan view of another optical system using a laser and including means to focus the laser beam to form four beams as lines with controlled energy densities;

FIG. 26 illustrates in diagrammatic fashion the effect of the focusing means of FIG. 25;

FIG. 27 is a side elevational view, partly in cross section, of radiation shielding means surrounding the heating zone;

FIG. 28 is a cross section of the optics for using a laser to obtain both external and internal heating;

FIG. 29 illustrates in somewhat diagrammatic cross sectional form the providing of a controlled fluid surrounding for the tubing during formation using the laser heating system of FIG. 25;

FIG. 30 illustrates in somewhat diagrammatic cross sectional form the providing of a controlled fluid surroundings for the tubing during formation using r.f. heating; and

FIG. 31 illustrates the use of supporting means for the tubing.

There is, of course, a great deal of prior art on floatzone melting of solid rods of various materials. See for example "Zone Melting" by William G. Pfann, John Wiley & Sons, Inc., New York, New York, 158, and U.S. Pat. No. 3,121,619. However, this technique has not previously been applied in any workable manner to the formation of tubings which presents particular problems not encountered in the processing of solid rods. British patent specification No. 1,226,473 mentions in passing that a laser beam may be focused at a point onto a tubing surface, but no workable method is taught which makes it possible to overcome the problems associated with tubing or to form refractory tubings of the character sought and achieved by the method of this invention. Those particular problems which are associated with tubings include the maintenance and adjustment of wall thicknesses, the maintaining of a desired cross section of tubing, the homogenization of the refractory materials, the optional formation of a single crystal, and the like. Tubings have also been pulled from molten material contained within a crucible as shown in U. S. Pat. No. 3,015,592. However, the physics, as well as the apparatus, of the floating zone process are completely different from the crucible-contained process.

There is, of course, considerable art on the drawing of glass tubings using dies and mandrels (see for example U.S. Pat. Nos. 3,620,704 and 3,672,201). However, such processes are not applicable to the formation of refractory tubings and particularly to forming tubings of materials having very high melting points or tubings of exceptionally high purity and/or in single crystal form.

Before describing various embodiments of the method and apparatus of this invention, it will be helpful to present the method generally with reference to FIG. 1 which is an enlarged longitudinal cross section of the molten zone forming section of the system. As a solid preformed tubing feed blank 20 (generally formed by pressing the refractory powder and presintering if necessary) is moved upwardly through a heating or melt zone 21 a ring 22 of molten refractory is formed and continuously, in effect, advances through the tubing in a downwardly direction, forming a refractory tubing 23. The dimensions of melt zone 21 are defined by the energy input distribution which in turn is determined by the nature of the input power, the thermal losses into the feed and drawing rods and the thermal losses to the surrounding environment.

The height h of the heating zone must be so controlled through the adjustment of these parameters, and the solid sections 20 and 23 of the tubings must be moved at such a rate as to always keep them joined through molten ring 22 which effects such joining through the forces of surface tension and gravity.

The solid preformed feed tubing 20 has an internal radius of r_(a-1), an external radius of r_(a-2), a wall thickness of t_(a) and a fractional density of ρ_(a). In like manner, the solid tubing 23 has an internal radius of r_(b-1), an external radius of r_(b-2), a wall thickness of t_(b) and a fractional density of ρ_(b), generally 100%. In order to establish a stable system permitting the continuous advancement of the molten ring 22, the mass flow rate crossing the solid-liquid boundary 24 between tubing feed section 20 and molten ring 22 and the mass flow rate crossing the liquid-solid boundary 25 between molten ring 22 and tubing section 23 must be equal. Since the velocities at which the tubing feed section 20 and the tubing section 23 are moved may be separately controlled, it is possible to adjust the wall thickness, and to some extent the outside diameter of the tubing formed, by moving the tubing section 23 at a greater or lesser velocity. The situation diagrammed in FIG. 2 shows how the wall thickness may be attenuated by moving section 23 faster than section 20.

Since the mass flow rate is equal to the product of factional density, cross sectional area and velocity, the required stable system is attained when

    π(r.sub.a-2.sup.2 -r.sub.a-1.sup.2)ρ.sub.a v.sub.a =π(r.sub.b-2.sup.2 -r.sub.b-1.sup.2)ρ.sub.b v.sub.b

where v_(a) and v_(b) are the velocities at which the tubing sections are moved. Assuming that the fractional densities of both tubes are substantially 100%, and that r_(b-2) is essentially equal to r_(a-2), then

    2πr.sub.a-2 t.sub.a ρ.sub.a v.sub.a ≃2πr.sub.b-2 t.sub.b ρ.sub.b v.sub.b.

By increasing v_(b) it is possible to decrease t_(b), or by decreasing v_(b) it is possible to increase t_(b), so long of course as the basic requirement is met that the two sections are continuously joined through the molten ring. Thus there is provided a way of controlling the wall thickness of the finished refractory tubing.

The inside and outside diameters of the final tubing are functions of zone height and dimensions of the feed tube. If there is no attenuation during the formation of the final tubing, the resulting tubing will generally have a somewhat small diameter and a greater wall thickness. With attenuation, it is possible to change the relationship between the outside diameter and wall thickness; but the outside diameter of the growing tube will always to equal to or smaller than the outside diameter of the feed tube.

It is, of course, within the scope of this invention to form refractory tubings having a range of cross sectional configurations in which the wall thickness may be uniform or nonuniform. Exemplary of a circular cross section of uniform wall thickness is tubing 28 of FIG. 3, and of an eliptical cross section with nonuniform wall thickness is tubing 29 of FIG. 4. The ultimate cross sectional configuration may be controlled in forming the feed tubing blank and to some extent by the design of the heating zone.

Although it is possible to start the growth of the final tubing by placing the feed tube in the melting zone so that melting is begun somewhere intermediate between the ends of the feed tube, it is usually more desirable to begin the melting at one end of the feed tube. To do this, it is necessary to bring one end of the feed tube located in the melt zone into contact with a contacting solid surface member, hereinafter referred to as a pick-up, which is affixed to a load-bearing rod such as the rod described in connection with FIG. 18. With the melting of the end of the feed tube and the formation of a melt in the contacting surface it is possible to make contact and "weld" the tubing to the pick-up. The pick-up may take any desirable form and may, in some instances, be a single crystal used in a seeding function to start the formation of a single-crystal tubing. Several exemplary forms of pick-ups are shown in FIGS. 5-17.

In FIG. 5 the pick-up 30 is in the form of an annular ring (or other suitable cross section) of any desired length which is brought into contact with a molten ring 31 formed on one end of the feed tubing 20 while the end of the tubing is in the heating zone and it heated by means diagrammatically represented by arrows 32. FIGS. 6 and 7 illustrate the use of a pick-up in the form of a flat plate 34 which, like the annular ring 30, is contacted with molten ring 31 in the heating zone. Subsequent to contacting the tubing molten ring 31 to the pick-up, the process of tubing formation is continued as described. When the flat plate 34 is used as a seed crystal, a joint 35 may be formed, as shown in FIG. 7, and later removed.

In FIG. 8, the pick-up is a rod 36, the end 37 of which is melted and joined to the molten end of feed tube 20. In some cases, the rod has been found to be a preferred form of surface contacting member since it may be used to form a stronger weld with the feed tube than the thin ring 30 or flat plate 34 of FIGS. 5 and 6.

FIGS. 9-17 illustrate the use of several embodiments of a pick-up in tubing form. This type of pick-up has been found to be the preferred embodiments in forming a single-crystal tubing. FIGS. 9-12 illustrate the use of a tubular pick-up in starting the formation of a tubing by the method of this invention. A preformed feed tube 38 having its upper edge 39 cut straight across is held in chuck 40; and a pick-up tubing 41 having a frustoconically configured contacting end 42 is held in chuck 43. These chucks 40 and 43 are affixed to load bearing rods 44 and 45, respectively. Separate means, not shown, are provided to impart translational and rotational motion to load-bearing rods 44 and 45 and through them to the sections of the tubing. Since each load-bearing rod has its own individual moving means the translational and rotational motions of the feed rod and of the pick-up can be separately controlled. Exemplary apparatus for moving such load-bearing rods at desired axial and angular velocities are described in U.S. Pat. No. 3,552,931. Since the load-bearing rods such as in the apparatus of U.S. Pat. No. 3,552,931 are moved by separate mechanisms, it is possible to achieve adjustments in wall thickness and tubing diameter through different rates of translational motion as well as uniformity in tubing size and homogenization of material through rotational motion.

As shown in FIG. 10, feed tube 38 is introduced into heating zone 21 to form an initial molten ring 46, the cross section of which, through the forces of surface tension, tends to approach a circle. Once this initial molten ring 46 is formed around the top of feed tube 38, the pick-up seed tube 41 is lowered to make physical contact with initial molten ring 46 as shown in FIG. 11. This leads to the welding of the pick-up seed tube to the initial molten ring 47 (FIG. 12) which is then advanced along the length of the feed tube to form the final tubing of this invention.

It is also within the scope of this invention to bring the pick-up tube (or rod) into contact with the feed tube prior to any heating, i.e., prior to the formation of the molten ring. It is also within the scope of this invention to move either or both the pick-up and feed tube to make the contact.

The relative sizes and configurations of the feed tube and pick-up tube must be such that once the initial molten ring 46 is formed on the end of the feed tube, the pick-up tube is able to make physical contact with it so that a molten ring is formed between the two which extends all the way around and all the way through the tubing.

In one preferred embodiment of the method of this invention, as indicated in FIG. 12, the feed tube 38 and pick-up tube, along with subsequently formed tubing attached thereto, are rotated in opposite directions while the molten ring 47 is advanced through the tubing. This counterrotation of the two solid sections on either side of the molten ring brings about a homogenization of the material in the tubing formed and minimizes any variations in heating intensity within the heating zone 21.

The relative size/configuration combination of the feed tube and pick-up tube may be realized in a number of different embodiments, exemplary ones of which are illustrated in fragmentary cross sections in FIGS. 9 and 13-17. For example, in FIGS. 9, 13 and 14, such uniform and complete contacting of the pick-up tubing with the initial molten ring on the end of the feed tube is insured by using a pick-up tube having an outside diameter, OD_(p), greater than the inside diameter, ID_(f), of the feed tube, the wall thickness of the pick-up tubing being essentially the same or somewhat less than that of the feed tube. In FIG. 13, both the feed tube 38 and the pick-up tube 41 have straight-cut contacting ends 39 and 48, while in FIG. 14, feed tube 38 has a contacting end 49 in which the tube wall is cut to have an inwardly slanting configuration. These Figures illustrate the contacting of the pick-up and feed tubes prior to the formation of the molten ring.

In the embodiment of FIG. 15, the feed and pick-up tubes have essentially the same cross sectional dimensions and the contacting ends of the feed tube and pick-up tube are cut to present complementary frustoconically configured faces 49 and 42, respectively. It is, of course, within the scope of this invention to use the reverse of the tubing cuts shown in FIG. 16.

In the embodiment of FIG. 16 the pick-up tube is sized to fit within the feed tube. This is acceptable so long as the increase in melt volume experienced by the molten ring 46 in its formation is sufficient to insure the desired contact with the end of pick-up tube 41. Finally, as shown in FIG. 17, it is also possible to use a pick-up tube which is larger in cross section than the feed tube. FIG. 17 also shows the use of a feed tube with a frustoconically configured contacting end 50.

Although FIGS. 9-17 have illustrated the feed tube as the lower tube and the pick-up tube as the upper tube, the reverse arrangement may be used since the forces of surface tension may be relied upon to hold the initial molten ring 46 onto the feed tube end. The optimum choice of pick-up tube configuration and location may be readily determined for any one refractory material and tubing size.

Many different sources of heat may be used to form the required heating zone and the selection of the heating means will usually depend upon the specific refractory material used. Such heat sources include, but are not necessarily limited to, infrared sources such as incandenscent filaments, arcs, imaged arcs and lasers; electron beams, gas discharge systems and direct r.f. coupling. FIGS. 18-27 illustrate exemplary heating systems to provide external and internal heating.

In the apparatus illustrated in FIG. 18, in which like reference numerals are used to refer to like elements in previously described drawings, means are provided for external heating and for moving the tubing while maintaining the heating zone and its heating means stationary. The feed tubing 20 in this case is moved upwardly through the heating zone 21 while the ends of the tubing are held in chucks 40 and 43. In the apparatus of FIG. 18 heating is done by means of an r.f. coil 51 inductively coupled to a graphite ring 52 (which in effect becomes an incandescent filament) through water-cooled copper electrodes 53, field concentrating ring 54 and a boron nitride insulator ring 55. A modification of the apparatus of FIG. 18 is suitable for making tubing of electrically conducting materials by direct inductive coupling of the feed tubing 20 with the field concentrating ring 54 or directly with the r.f. coils as shown in FIG. 22. The feed tubing material must, however, be electrically conducting at appropriate frequencies and temperatures; and in such a system the frequency of the r.f. heating system must be adjusted with respect to wall thickness.

The apparatus of FIG. 18 may be operated in other modes as well. The load-bearing rods 44 and 45 may be rotated at the same or different angular velocities in the same or opposite directions to insure very even distribution of heat into the tubing, homogenization of the molten material and hence a uniformly heated molten ring 22. In another operational mode, assuming that the tubing is moving upwardly, rod 45 may be moved upwardly at a rate greater than that at which rod 44 is moved upwardly, thus resulting in a final tubing wall thickness less than that of the preformed feed tubing. Making the same assumptions as stated above, rod 45 may be moved upwardly at a rate less than that at which rod 44 is moved upwardly, thus resulting in a final tubing wall thickness greater than that of the preformed tubing. It is also of course, possible to reverse the direction of travel of the tubing and to make corresponding adjustments in the rates at which rods 44 and 45 are moved.

If, instead of moving the entire tubing, the heating means are moved in FIG. 18, then by holding both solid tubing sections 20 and 23 stationary, or by rotating them without effecting any translational motion, the wall thickness of the final tubing will be essentially that of the feed tubing provided the fractional density of the feed tubing approached 100%. However, in this general mode of operation it is also possible to adjust the final tubing wall thickness by imparting motion in either direction to that pulling rod associated with the solid tubing section from which the molten ring is moving. As an example, if the heating means of FIG. 18 are moving downwardly, motion may be imparted to feed tube 45--upwardly to attenuate wall thickness and downwardly to increase wall thickness.

In the apparatus of FIG. 19, means, located internally of the tubing, are provided to augment and direct the heating from an externally provided heating means. FIG. 19 also shows the reversal of the direction of tubing travel. The heating means are shown to comprise an incandescent filament 60 which may be in the form of a strip having a circular opening 61 defining the heating zone. The filament 60 is connected to two water-cooled electrodes 62 which in turn are connected to a suitable power supply, not shown. For alumina tubing the filament 60 may typically be formed of graphite to obtain temperatures between 2400° and 2800° C. and of tungsten to obtain temperatures of about 3000° C. Because graphite and tungsten readily oxidize at these temperatures it is necessary to provide a nonoxidizing atmosphere surrounding the tubing and the heating zone. In the apparatus of FIG. 19 an essentially fluid-tight housing 63 provides a volume 64 which may be evacuated or charged with a gas inert to the melt and electrodes through a valve-controlled line 65. The mechanism for imparting translational (and rotational) motion to the hollow load-bearing rods 66 and 67 described in U.S. Pat. No. 3,552,931 is designed to operate with a fluid-tight housing which may be evacuated or pressurized, and provides the necessary sealing means 68 and 69 which permit the actual driving mechanism for the load bearing rods 66 and 67 to be located external of housing 63. Extending through rods 66 and 67 and chucks 70 and 71 are two radiation shield rods 73 and 74 which are positioned within the internal volume 72 of the tubing. These rods are stationary and are so positioned and configured as to direct the heat radiated by the internal wall of molten ring 22 back to the molten ring surface, thus preventing the unwanted heating of the solid tubing sections on either side of the ring and providing for maximum utility of the heat furnished in the heating zone.

In FIG. 20, in which like reference numerals are used to refer to like elements in FIG. 19, external heating may be furnished to heating zone 21 by any of the means herein described. Separate internal heating is provided by an arc 80 established between electrodes 81 and 82 connected to a suitable power supply not shown. Likewise in the embodiment of FIG. 12, internal heating developed by a filament 85 connected between two terminals 86 and 87, which are in turn connected to a suitable power supply, may be used to augment external heating supplied to heating zone 21.

In the embodiment of FIG. 22, the tubing is held stationary in a mounting ring 90 placed on a platform 91. A refractory rod 92, (such as of graphite) is centrally positioned within internal tubing volume 72 and an r.f. coil 93 is mounted through suitable insulation members 94 and 95 to a load bearing rod which extends down through an opening in platform 91. That graphite rod section corresponding to the position of external r.f. coil 93 is directly coupled to the r.f. coil, thus generating heat internally within the tubing.

As will be apparent from the following discussion of the apparatus of FIGS. 23-29 showing the use of a laser, this type of heating means has distinct advantages for practicing the method of this invention. Where incandescent heating systems emit a large part of their radiation in a wavelength range to which many of the refractories in their molten state are transparent, the laser can be chosen to avoid this difficulty. Incandenscent systems may pose problems of heat transfer to the tubing but laser energy can be directed to avoid such problems. Laser energy has no characteristic temperature of its own, and thus there are no upper temperature limitations; and the use of lasers imposes few restriction on the atmosphere in which the tubing is formed. Finally, as will be seen from the description of FIGS. 23-29, the use of a laser permits visual observation of the process.

FIG. 23 is a schematic representation of an optical system using a laser. Radiation from laser 100 is passed through a beam expander 101 typically formed of lenses 102 and 103 and an aperture 104. The expanded beam strikes plane mirror 105 and is reflected on plane mirrors 106 and 106A positioned such that each of these mirrors receives a half-circle of the beam. The semicircular beam from mirror 106 is interrupted by plane mirrors 107 and 108 positioned in the vertical plane such that each mirror 107 and 108 receives a quadrant of the beam. In like manner the semicircular beam from mirror 106A is interrupted by plane mirrors 109 and 110 also positioned in the vertical plane such that mirrors 109 and 110 also receive a quandrant of the beam. A quandrant is reflected from plane mirror 107 to a focusing mirror 111 which in turn is so positioned to focus the reflected beam on the molten ring located in the heating zone 21. In like manner the quadrant of the beam striking mirror 108 is reflected and focused by mirror 112, the quadrant striking mirror 109 is reflected and by mirror 113 and the quadrant striking mirror 110 is reflected and focused by mirror 114. It is possible to divide the laser beam into two or more beams for focusing in a similar manner. However, the beams should be symmetrically arranged to provide a uniform heating zone.

FIG. 24 which is a cross section through the optical system shows the manner in which the plane mirrors are hung from a jig 115, by adjustable supports 116, to attain the desired vertical positioning which permits the splitting of the laser beam and its refocusing in four beams at the heating zone. The jig and focusing mirrors can be mounted in any suitable manner and the tubing is moved axially as shown in FIG. 18.

FIG. 25 is an optical diagram for another embodiment of the apparatus of this invention using a laser heat source. The features of this apparatus include means to form the laser beam into an annulus, means to split the beam then into two annuli, means to redivide both annuli into half annuli and means to focus the annuli to form beams in line configurations having energy density distributions particularly suitable for heating a portion of a tubing surface.

In the apparatus of FIG. 25, the radiation beam 150 from laser 100 is first directed through a beam-expanding means 151 which comprises a spherical mirror 152 and a spherical mirror 153. By proper choice of focal length of the two spherical mirrors, it is possible to expand the laser beam by a factor of two, four or greater. A rotating Dove prism 154 is used to form the expanded beam 155 into a beam 156 having an annular configuration. Dove prisms are known and described in the literature. (See for example "Modern Optical Engineering" by Warren J. Smith, McGraw-Hill Book Co., New York, New York, 1966, page 87.) By focusing expanded beam 155 through Dove prism 154 above or below the optical axis of the prism and by rotating the prism about its axis by suitable means such as motor 157, it is possible to form the laser beam into an annular configuration.

The annular beam 156 is then passed through a beam spliter 158 which typically comprises a water-cooled, coated GaAs window 159 and a front surface mirror 160. The coating thicknesses on the surfaces of window 159 are designed to reflect 50% of the beam from the front surface of the window to form annular beam 161 and to have no reflection losses from the rear surfaces. Half of the incident beam 156 is transmitted to mirror 160 where it is reflected as annular beam 162 parallel to annular beam 161. These two annular beams are each then split to form two half-annular beams and the resulting four half-annular beams are then focused as line beams onto that portion of the tubing surface which is within the melt zone 21.

Beginning first with annular beam 161, it is directed onto a semicircular mirror 165 which is so positioned as to permit one-half of the annular beam in the form of a half-annulus beam 166 to strike plane mirror 167 for reflection to a variable focal length cylindrical mirror 168 which focuses the beam as a line (line beam 169) onto the tubing to form molten ring 22. As in the case of the apparatus of FIG. 24, the optical elements of FIG. 25 are mounted on or suspended from a support base 170. As is well-known in optical engineering, the elements are so spatially positioned to make these optical paths possible. In FIG. 25, the line beam 169 is shown as a dotted line to show that it is distinct from half-annulus beam 166. However, it will be appreciated that these two beams are in two different planes and not side-by-side. In a similar manner, the half-annulus beam 172 is reflected by cylindrical mirror 173 which returns a line beam 174 to be directed onto the tubing to form molten ring 22. Similarly annular beam 162 is split into two half-annulus beams 175 and 176 by semicircular mirror 177; beam 175 is focused by plane mirror 178 onto cylindrical mirror 179 to be transformed into line beam 180; and half-annular beam 176 is focused by cylindrical mirror 181 into line beam 182.

A toroidal radiation shield 185 having four ports 186 to permit passage of line beams 169, 174, 180 and 182 is provided to radiate energy reflected by the molten tubing surface back to the surface to concentrate and conserve the energy used in melting the tubing. This radiation shield is described in detail in connection with the discussion of FIG. 27.

FIG. 26 is presented to describe the advantages realized by the optical system of FIG. 25. When a half-annulus laser beam, such as beam 166, is focused into a line, such as line beam 169, it can be shown to have an energy density curve which peaks at or near each end of the line beam. When a beam of radiation strikes a circular surface such as that of the tubing, the amount of energy absorbed by the surface decreases as the angle θ, formed between a line drawn normal to the tubing surface and the line of the beam, becomes greater than zero. It thus becomes apparent that when a line beam having a constant energy density along its length strikes a circular surface, the portions of the surface receiving radiation from the ends of the beam will absorb less of it. However, by focusing a half-annulus laser beam down to a line beam, it is possible to form a beam having increased energy densities towards it ends just where such added energy is required to overcome the differential in energy absorption as described. This is shown in FIG. 26. Hence the apparatus of FIG. 25 provides means to heat the tubing in the melt zone more uniformly around the entire periphery of the tubing, since each of the four line beams have this unique energy density distribution. It will be appreciated that the term "line beam" is used to designate a beam which has, in fact, some height, the height being controlled through the focusing of the cylindrical mirrors.

FIG. 27 illustrates an exemplary radiation shield formed to define a toroidal surface 190 surrounding the heating zone 21 and curved to return radiation reflected by or emitted from the molten ring back to the molten ring. Normally the radius of curvature R will be equivalent to twice the focal length of the mirror defining surface 190. This mirror is coated to produce a surface which is highly reflective to infrared radiation, such as bright gold. The toroid 191, the inner surface of which serves as mirror 190, is machined as an upper half 192 and lower mating half 193. Each of these toroid halves is also machined to have an integral joining and support means which takes the form of a shorter clamping section 194, formed of upper half 195 and lower half 196, and a longer section 197 formed of upper half 198 and lower half 199, providing means to clamp, cool and align the toroid halves. The support means must be of such dimensions that it clears ports 186 in the toroid. Clamping is accomplished with a series of bolts 200 and nuts 201, and alignment of the toroid halves and support means halves is achieved and maintained through a series of dowel pins and pin holes (not shown) in the longer sections 198 and 199 of the support means. The entire radiation shielding means 185 is preferably machined from copper and is cooled by circulation of a cooling liquid, e.g., water, through the longer supporting section halves 198 and 199. The cooling liquid is introduced through an inlet conduit 202 into one or more passages 203 and withdawn through passage 204 which terminates outside supporting section half 195 as an annular outlet conduit 205 surrounding and concentric with inlet conduit 202. Suitable sealing rings such as O-rings 206 and 207 provide adequate sealing between the mating passages in section halves 198 and 199. The radiation shielding means is supported by the base support 170 (FIG. 24) through the cooling liquid conduit 205 and hence the upper section of the shielding remains fixed with respect to the other optical components. It is therefore a simple matter to remove and replace the lower section while maintaining the desired alignment.

It will, of course, be appreciated that the construction illustrated in FIG. 27 and described above is only exemplary of one possible configuration and assembly of the radiation shielding means used. Therefore, it is within the scope of this invention to use any suitably-configured radiation shield (e.g., a spherically-shaped one) assembled in any appropriate manner.

Since it will normally be advantageous to use a single type of laser for forming tubings of different material, the use of the radiation shielding is particularly valuable when working with materials which reflect an appreciable amount to the laser's radiation.

The apparatus of FIG. 28 uses a laser for achieving external and internal heating. The collimated radiation beam from a laser (not shown) is directed onto flat reflectors such as annular mirror 120, and central mirror 122. Positioned around the outer wall of the tubing in an annular focusing reflector 125. A reflector 126 which has a conical-like configuration is positioned internally of the tubing at the same level as annular reflector 125. In this arrangement, the radiation is directed against both the outside and inside of the tubing wall over sharply defined areas.

FIG. 29 illustrates the providing of a controlled atmosphere around the tubing during formation using laser heating. As in the case of the system of FIG. 19, the housing 63 may be evacuated or charged with a gas of the desired character, e.g., reducing, oxidizing or inert. The laser optics are those shown in FIG. 25, the same reference numerals being used to identify the optical elements shown for purposes of illustration. Housing 63 has a plurality of windows, such as 210 and 211, a window being provided for each beam of laser energy striking the tubing surface.

In the apparatus of FIG. 30 means are provided to control the atmosphere around the entire tubing forming system and also around the part of the system where the tubing is actually formed by passing through the heating zone. In handling some materials which contain one or more volatile components in the melt stage such as arsenic and phosphorus, it is necessary to encapsulate the melt and in some cases to work under pressures which are greater than the vapor pressure of the volatile component. In some other circumstances, it may be advantageous to be able to reduce the forces on the molten ring in forming the tubing.

The tubing forming system of FIG. 30, which uses the means for continuously supplying a feed tubing of FIG. 18, is enclosed in a fluid-tight chamber 63 such as that of FIG. 19, which may be pressurized if desired. The load bearing rod 44 on which a modified chuck 130 is mounted is introduced through the bottom of a vessel 131 containing a liquid encapsulant 132 in which is embedded an r.f. coil 133 exemplary of a suitable heating means. If the encapsulant is also to be present within the tubing, a plurality of ports 134 may be drilled in chuck 130 to permit the liquid encapsulant to flow upwardly into tubing volume 72. In the modification of FIG. 30, vessel 131 is shown to be supported on a ring support 135 within fluid-tight housing 63.

The liquid encapsulant should have a density which approximates the density of the refractory in molten form and which is inert to the melt as well as to all other material it contacts. The liquid encapsulant should also of course be compatible with the heating means and not interfere with its function. For example, in the apparatus of FIG. 30 which uses an r.f. heater the liquid encapsulant should not be electrically conducting at the frequencies employed in the heater. Exemplary of encapsulants which are suitable are boric oxide, barium oxide and these oxides in admixture with barium chloride and sodium fluoride.

The use of an encapsulant used as shown in FIG. 30 may have advantages for certain systems. For example, a liquid encapsulant, which has a density that approximates the density of the molten ring, reduces the body forces on the molten volume. Thus the surface energy is that of a liquid-liquid interface rather than that of a gas-liquid interface and hence the tendency to neck off is reduced. Such a liquid encapsulant acts as a heat sink, thus increasing the rate at which the growing tubing section may be solidified.

Finally, it may be desirable in some instances to provide some type of confinement or support for the tubing during the process, particularly if the refractory material being used does not lend itself to the formation of an adequately self-supporting feed tubing configuration. The modification of the apparatus shown in FIG. 31 provides an outer supporting member 140 of any desired cross section and a mandrel 141 which corresponds in size and cross section to the inner wall of the tubing. A seed crystal 142 may be placed either at the bottom or top of the tubing, depending upon the direction in which the tubing is to be moved. It is, of course necessary to construct the outer supporting member and inner mandrel of a material which has a higher melting point than that of the refractory from which the tubing is to be formed and which is inert to it. If an r.f. coil is used as the heating means, if the outer supporting member 140 and mandrel 141 are constructed of a suitable material such as graphite and if the wall thickness of supporting member 140 is relatively small, then some internal heating of the tubing can be achieved through direct r.f. coupling of the mandrel.

From the drawings presented and from the description and discussions of these drawings, it will be apparent to those skilled in the art that a number of different heating means and combinations of heating means may be used. In a like manner, a number of different arrangements for effecting the relative motion of such heating means and the tubing are possible. Moreover, any of the apparatus embodiments illustrated or discussed may be located within a chamber in which the fluid surroundings may be controlled during the formation of the tubing. Exemplary of apparatus which provides such a chamber is the pressure- and temperature-controlled furnace described in U.S. Pat. No. 3,639,718 and assigned to the same assignee as the present application. The various modes of operation described for the apparatus of FIG. 18 are, of course, applicable to the other apparatus embodiments and modification.

The method and apparatus of this invention may be used to form refractory tubing from any refractory material which is capable of existing in the liquid state and which can be formed into a tubing blank. In addition to alumina, such refractories include, but are not limited to, oxides such as zirconia, titania, thoria, ytteria and the like, carbides such as titanium carbide, borides such as titanium boride, intermetallics such as gallium arsenide, ternary compounds such as HgCdTe, pseudobinary compounds such as AlAs-GaAs, and elements such as boron and silicon. Both crystalline and amorphous materials may be used. For optical applications, it is generally necessary to use materials of extremely high purity. For other applications it may be desirable to have dopants, such as titanium or other additives present in minor amounts.

The feed tube blank used to produce the polycrystalline or single crystal tubing may be prepared by any suitable technique such as for example by slip casting, or by pressing the refractory in powder form (with or without a heat-removable binder) under sufficient pressure to form a self-supporting structure. This pressed structure may, if desired, be partially sintered to enhance its structural strength. Tubing blanks of presintered materials, e.g., Al₂ O₃, are available commercially. (See also, for example, "Introduction to Ceramics" by W. D. Kingery, John Wiley & Sons, Inc., New York, 1960, particularly Chapter 3 on "Forming Processes" which describes in detail such forming processes as powder pressing, extrusion, slip casting and sintering.) In view of the well developed art in the formation of ceramic or refractory bodies, the choice of such parameters as particle size, binders, and density of the material making up the feed tubing is within the skill of the artisan in this field; and the choice of the method by which the feed tubing is formed is also within his skill. As noted above in the description of FIG. 31, it may under some circumstances be desirable to provide some physical support to the feedtube blank in the form of an outer supporting member and centrally positioned mandrel.

If a single crystal tubing is to be formed using a seed crystal, the quality of the seed crystal must be consistent with the quality desired of the finished tubing. It is also necessary that suitable pretreatment of the seed crystal is effected to eliminate any work damage sustained by the seed crystal in shaping and/or cutting. Techniques for providing suitable seed crystals in any desired form are well known.

The temperature attained within the heating zone must, of course, be that which is sufficient to melt the tubing. Although somewhat higher temperatures can be tolerated, the temperature should be maintained somewhat below that level which would cause vaporization or boiling off of the molten material under the environmental conditions being used. Thus temperature range, which is readily determinable from existing physical data, will depend upon the refractory material from which the tubing is formed.

The height of the travelling molten ring is controlled by well-known physical factors, i.e., the existing temperature gradient which is a function of the thermal conductivity of the tubing material and the environment surrounding the tubing. As noted above this environment may include the ambient atmosphere, an inert pressurized gas with or without a liquid encapsulant, or a gas providing a special type of atmosphere, e.g., reducing or oxidizing. In addition, the use of radiation shielding to return reflected radiation to the molten zone represents another factor in the environment. The growth rate of the finished tubing is, of course, equal to the withdrawal rate, v_(b).

Because the method of this invention involves the melting of the tubing, it permits the selection of any desirable atmosphere to achieve such results as purification of the refractory material and adjusting the valence state of any additives. For example, if the process is carried out in an oxidizing atmosphere it is possible to convert to or maintain a titanium dopant in its higher valence state, i.e., Ti⁺⁴. Other types of environments may, of course, be used to attain other desired results.

The method of this invention may be further illustrated by the following examples which are meant to be illustrative and not limiting.

A two-inch length of a commercially available sintered polycrystalline alumina tubing (0.999% Al₂ O₃) having an outside diameter of 0.24 inch and an inside diameter of 0.162 inch (sold by McDanel, Inc., as AP35) was placed in an apparatus constructed as shown in FIG. 18. The incandescent filament used was a graphite washer. The top and bottom pulling rods were moved at 3 inches per hour while the heating zone was maintained stationary. At the completion of zone melting the tubing had an outside diameter of 0.226 while the inside diameter remained essentially the same. A second tubing sample of alumina tubing having an outside diameter of 0.084 inch and inside diameter of 0.053 inch was treated in the same manner. After zone melting was completed, it had an outside diameter of 0.082 and an inside diameter of 0.049.

The reduction in wall thickness from 0.039 to 0.032 inch in the first example illustrates the densifying of the alumina into a structure of reduced porosity; while the decrease in outside diameter and inside diameter with an accompanying shortening of the tube length confirmed the densifying of the second sample.

A preformed open-ended alumina tubing (sold as Al-23 by Degussa Inc.) was externally ground down to have an outside diameter of 0.375 inch. It had an inside diameter of 0.315 inch and wall thickness of 0.030 inch. The alumina in the tubing as purchased had a density of between 3.7 and 3.9 grams per cm³. This tubing was treated in an apparatus such as shown in FIG. 18 in an argon atmosphere. The tubing was affixed by chucks between upper and lower load-bearing rods (e.g., 43 and 42 of FIG. 18) and the upper pulling rod was moved upwardly at a speed which was 1.25 times the speed at which the lower rod was moved upwardly. The tubing was not rotated. Molten ring formation was started part way up the tubing and the growth rate (ring movement) was 5.2 inches/hour. Inasmuch as the upper pulling rod was moved at a higher speed than the lower rod, there was attenuation of the tubing, as evidenced by the fact that the finished tubing had an outside diameter of 0.332 inch, an inside diameter of 0.302 inch and a wall thickness of 0.015 inch. The finished tube was translucent and had a density approaching theoretical for Al₂ O₃.

An open-ended tubing, commercially available from Degussa Inc. as Al-23, was processed in the apparatus shown in FIG. 16. The tubing as purchased had an outside diameter of 0.401 inch, an inside diameter of 0.285 inch and wall thickness of 0.058. A CO₂ laser (10.6 μm radiation) was used as the heat source and a solid rod of Al₂ O₃ was used as the starting contacting member as shown in FIG. 8. The load-bearing rods were rotated at 95 rpm, and the speed of the upper one was 1.6 times that of the lower one to achieve attenuation. Growth rate was 7 inches/hour and the environment was ambient air. The resulting tubing was translucent and had a density approaching theoretical for Al₂ O₃.

A single crystal alumina tubing was formed by the process illustrated in FIGS. 9-12. A tubing of 99% pure alumina, formed by cold pressing to have a fractional density of 97%, and having an outside diameter of 9.15 mm with a wall thickness of 1.01 mm was used as the feed tube. A single crystal tubing of alumina having an outside diameter of 7.1 mm, a wall thickness of 0.5 mm and a frustoconical contacting end served in the dual role of pick-up tube and seed crystal. A CO₂ laser using 415-430 watts power was used to form the heating zone. The optics were essentially those illustrated in FIGS. 25 and 27, with the exception that the radiation shielding was spherically shaped.

Once the molten ring was formed in the contacting feed and pick-up tubes, the feed tube was moved upwardly at 3.43 inches per hour and the solid tube section attached to the pick-up tube was pulled at a rate of 6.06 inches per hour. Simultaneously with these translational motions, the feed tube was rotated in one direction at 490 rpm and the solid section of the tube attached to the pick-up tube was rotated in the opposite direction at 385 rpm.

The finished single crystal tubing had an outside diameter of 7 mm and a wall thickness of 0.7 mm. It was transparent and free from any cracking.

The method of this invention thereby provides for upgrading of a refractory tubing structure and, if desired, for the forming of such a structure in the form of a single crystal, a form which possesses operational advantages when the resulting crystalline tubing is used as an enclosure for high-pressure lights.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 

I claim:
 1. A method of forming refractory tubing, comprising the steps of(a) providing a preformed tubing blank of a refractory material as a feed tube in the form of a first solid section; (b) providing around said tubing blank a concentrated heating zone of laser energy of substantially uniform intensity thereby to initially form on one end of said tubing a molten ring of said refractory material by(1) expanding a laser beam; (2) splitting the resulting expanded laser beam into a plurality of laser beams essentially equally spaced around the surface of said tubing, and (3) focusing each of said plurality of said laser beams onto said surface of said tubing by forming each of said laser beams into a half-annular configuration and focusing the resulting half-annular beams onto said tubing in a manner to adjust the energy density distribution of each of said beams to essentially equalize the absorption of laser energy around the entire surface of said tubing within said molten ring; (c) bringing a pick-up member as a second solid section into contact with said molten ring within said heating zone thereby to attach said pick-up member to said feed tube through said molten ring; (d) controllably effecting relative translational movement between said solid sections and said heating zone thereby to advance said molten ring through said tubing; and (e) controlling the characteristics of said heating zone to maintain said molten ring at an essentially uniform height and extending completely through said tubing, said height being such that the forces of surface tension and gravity maintain said molten ring connected in a substantially stable conditiou to said solid sections on either side of said molten ring of said tubing.
 2. A method in accordance with claim 1 wherein said step of forming said laser beams of half-annular configuration comprises expanding a laser beam, imparting an annular configuration to the resulting expanded beam, splitting the annular beam into at least two annular beams.
 3. A method in accordance with claim 1 wherein said focusing each of said laser beams of half-annular configuration includes focusing said beams into line beams. 